1. Field of the Disclosure
This disclosure relates to regulating voltage of solar arrays to optimize power generation. Particularly, this disclosure relates to regulating voltage of solar arrays to optimize power generation for space applications.
2. Description of the Related Art
Optimal power extraction from an illuminated solar array occurs when the operating array voltage of the solar array is at the peak power point. This array voltage at the peak power point is typically termed the Vmp, the voltage at which the maximum solar array power is generated. Typically, the Vmp is dependent on the age, season and temperature for a given solar array. When a solar array is used in a satellite, the solar array temperature frequently changes due to orbital variation and seasonal changes. The space radiation environment degrades the solar array which affects Vmp. Thus, as the satellite solar array ages, season and temperature change, so does its peak power point, Vmp. On the other hand, the various electrical systems of the satellite are typically designed to operate at a substantially fixed voltage. Accordingly, if the solar array is directly coupled to the electrical systems, it is necessary for the solar array voltage to be similarly fixed at the electrical system voltage level (known as the “bus voltage”) to protect the electrical systems. Because the satellite electrical systems do not change their operating voltage as the solar array Vmp changes, it is not possible to extract the maximum solar array power from a solar array at all times. Thus, power generation for the solar array is not optimized.
Different techniques have been employed to address the described power generation inefficiencies. For example, conventional satellites may employ a voltage regulator for the solar array described as a peak power tracker. Peak power trackers set the solar array voltage at an input and deliver a power voltage at an output. Thus, peak power trackers can improve solar array power extraction by continuously operating the solar array at the peak power point. However, it is necessary to regulate the output power voltage. In addition, these peak power trackers usually sense a combination of voltages and currents in order to determine the solar array operating voltage that results in maximum power extraction. Alternately, the solar array may simply designed to be larger than otherwise necessary in order to compensate for inefficient power extraction resulting from unoptimized solar array voltage set to match the bus voltage. A typical conventional satellite solar array may employ a combination of these techniques to achieve the system solution.
A typical conventional peak power tracker system employs a battery-on-bus topology where the power bus is connected to the battery. A pulse width modulator (PWM) regulator (buck or boost) is coupled between the solar array and the battery. By sensing the solar array current and voltage (need data processing to determine the voltage and current to produce peak power) and varying the pulse width or duty cycle of the PWM regulator, peak solar array power is extracted and stored in the battery to support the power bus. A conventional peak power tracker employs complex circuitry to determine peak power point because a data processing capability is required to determine the product of voltage and current calculated to determine peak power point. If the battery is close to being fully charge, duty cycle can be changed to reduce current. In this case, the battery sets the bus voltage.
A commonly used solar array regulator is a shunt regulator which regulates the bus to a fixed voltage by shunting excess solar array current. A shunt regulator can be fully passing, fully shunting or partially shunting and partially passing to match the load demand. The solar array operating voltage is set to the predicted peak power point only at end-of-life (EOL). Therefore a shunt regulator does not extract peak solar array power at beginning-of-life (BOL).
A conventional peak power tracker system that is coupled to the batteries gives rise to another problem because the peak power tracker output is unregulated. Thus, to obtain a regulated bus voltage a second stage DC-DC converter is needed coupled between the peak power tracker output and the power bus. This would increase spacecraft mass and further reduce overall power conversion efficiency.
In any case, the conventional techniques for addressing the described power optimization problem typically introduce a relatively high insertion loss between the solar array and the power bus. Such conventional solutions insertion loss results in power losses estimated at approximately 3% to 5% or even higher if a second stage DC-DC converter is used. Accordingly, although peak power trackers extract some additional solar array power, their benefit is diminished as a result of their own functional losses.
In view of the foregoing, there is a need in the art for apparatuses and methods for regulating the voltage of solar arrays in order to optimize solar array power generation. In addition, there is a need for such apparatuses and methods to optimize solar array power generation with reduced functional losses as compared with conventional solutions. There is particularly a need for such systems and apparatuses in space applications. There is further a need for such apparatuses and methods in space applications to optimize solar array power generation at beginning of life, various seasons and at cold temperature. There is still further a need for such apparatuses and methods in space applications to provide additional power during orbit raising employing electric thrusters. These and other needs are met by the present disclosure as detailed hereafter.